C-reactive protein (CRP) is a major human acute phase protein and a component of the innate immune response. Its serum concentration is increased during inflammatory states, persists for the duration of the inflammatory process and returns to its normal low concentration following subsidence of inflammation. While CRP is felt to play a significant role in inflammation and host defense, the mechanisms by which CRP exerts its effects are unclear. In vitro, CRP binds to phosphocholine (PCh) moieties and can then bind to complement C1q and activate the classical complement pathway. In addition, binding of CRP to phagocytic cells via Fc receptors, with a variety of functional consequences, has been described. Recent publication of the crystal structures of CRP has provided insight into the amino acids that mediate binding of CRP to PCh, to Fc receptors, and to C1q, permitting generation of CRP mutants incapable of binding to PCh and to Fc receptors, as well as incapable of activating complement. Most known functional activities of CRP, in vitro, are associated with ligand-binding and subsequent complement activation or phagocytosis. Accordingly, we will employ such mutants to define the roles of binding to PCh and Fe receptors, and of complement activation in 2 model systems: a) the protective role of CRP in bacterial infections and b) the putative role of CRP in the pathogenesis of atherosclerosis resulting from its ability to bind to enzymatically-degraded LDL (E-LDL). Our specific aims are: 1.To precisely define the ligand-binding sites on CRP required for binding to PCh, FcR and C1q and to generate mutants lacking these critical binding capabilities. 2. To define the role of these 3 binding capabilities in the protective effects of CRP in infection with Streptococcus pneumoniae, known to bind to CRP, and Salmonella typhimuriurn, which does not. We hypothesize that both complement activation and phagocytosis will be found to be involved in CRP-mediated protection of mice from bacterial infections. 3) To define the role of the 3 binding capabilities of interest on CRP-E-LDL interaction, and the role of such interaction in the pathogenesis of atherosclerosis. Our working hypothesis is that all 3 binding sites participate in the pathogenesis of atherosclerosis, by binding to E-LDL and initiating complement activation and uptake of E-LDL by macrophages. We will also determine the effects of injecting wild-type and mutant CRPs on the size of the atherosclerotic lesions formed in ApoE knock-out mice. These studies will provide substantial insight into the mechanisms by which this ancient protein may contribute to host defense, or alternatively, to pathogenesis of disease.